The invention concerns a method for superconductingly connecting two or more wires, each comprising at least one filament that contains MgB2 or a mixture of Mg and B, wherein the superconducting connection is realized by exposed end regions of the filaments via an MgB2 matrix.
A method of this type is disclosed in U.S. Pat. No. 7,226,894 B2.
Superconducting wires, through which an electric current can flow without loss, are used in a plurality of ways, in particular, for high-field magnet coils. Superconducting materials thereby become superconducting only below a material-specific transition temperature TC such that superconducting materials must be cooled for technical use. One of the most important superconducting materials in the technical field is NbTi which has a transition temperature of approximately 10K. Since this low transition temperature requires cooling with cryogenic helium (boiling point approximately 4K), NbTi is relatively expensive to use.
In 2001, superconductivity was discovered in the Mg—B system, which enables relatively inexpensive cooling without cryogenic helium with a transition temperature of approximately 39K for MgB2. For this reason, the operating costs of technical systems could be reduced and the ease of maintenance be increased. MgB2 also has a higher critical magnetic field Bc2 compared to NbTi. For this reason, the superconducting system MgB2 is currently the subject of numerous research and development activities.
EP 1 390 992 B1 (DE 602 13 688 T2) describes a method for producing MgB2, wherein a porous preform is produced from mechanically activated crystalline boron powder. The preform is heated together with metallic magnesium in a container under inert conditions. Liquid magnesium thereby flows through the porous preform and reacts to form MgB2.
One difficulty with the technical use of the MgB2 superconducting system is the provision of superconducting connecting points (“joints”), in particular, between two MgB2 superconducting wires. The quality of the superconducting connecting points thereby delimits the efficiency of the overall superconducting system.
U.S. Pat. No. 6,921,865 B2 discloses a joint of two superconducting wires, wherein exposed NbTi filaments of the superconducting wires project into bulk MgB2 powder and are compressed therewith.
U.S. Pat. No. 7,226,894 B2 discloses a connection between two MgB2 superconducting wires, wherein the MgB2 filaments are exposed at the ends of two superconducting wires and are overlapped. The exposed ends are then heated together with a mixture of boron powder and magnesium powder, wherein boron and magnesium react to form MgB2. The generated MgB2 superconductingly connects the two wires. The generated MgB2 is, however, highly porous and the resulting current-carrying capacity of the joint is relatively small.
DE 10 2006 020 829 A1 describes a superconductor connection between superconducting wire leads which can, e.g. be made from MgB2. The freed ends of the wire leads are thereby disposed in a bushing containing MgB2 bulk powder. The bulk powder is subsequently pressed together with the bushing.
It is the underlying purpose of the invention to improve the quality, in particular, the current-carrying capacity and critical magnetic field strength of a superconducting connection of MgB2 superconducting wires.